Electrostatic coating of electrically conducting substrates such as metal objects is well known. For example in certain paint spraying processes, paint is electrically charged and droplets of paint are sprayed onto an earthed metal object. Such methods have been successful in obtaining a uniform coating on the substrate.
Electrostatic coating of electrically non-conducting substrates, and pharmaceutical tablet cores in particular, is more difficult. There have been proposals for electrostatic coating of tablets for many years. For example, GB 1 075 404 proposes an apparatus for coating tablets in which an atomiser is used to spray finely divided particles of a coating solution onto tablets in a high potential field. The coating is dried using, for example, an infra-red heater. Such proposals have not however been practised on any substantial commercial scale and the coating of pharmaceutical tablet cores is most commonly carried out as a batch process by applying a liquid coating in a revolving drum. The liquid coating material may of course be supplied in some cases in powder form but, if so, it is then dissolved or dispersed in a liquid prior to application. It is not, therefore, applied to the tablet core in powder form.
It is in many ways easier to apply a liquid rather than a powder coating to the core of a pharmaceutical tablet. It is difficult to obtain adhesion of the powder to the tablet and in order to secure the coating to the core, the powder must be transformed into a film without damaging the tablet core, which usually will include organic materials. Furthermore an even coating is required and it is difficult to obtain an even coating of powder on a tablet core.
When a liquid coating is used, the coating must be dried. Theoretically such drying could in some circumstances be carried out at room temperature but in commercial practice it is important, for example because of the rate at which the process must be carried out, to heat the tablets and that is expensive because of the large input of energy required to vaporise the solvent used in the liquid coating. Another disadvantage of liquid coating is that it cannot be used for coating materials that are not soluble or suitably dispersible in a usable liquid, preferably water.
WO92/14451 is concerned with electrostatic powder coating of pharmaceutical tablets and describes and illustrates a process in which the cores of pharmaceutical tablets are conveyed on an earthed conveyor belt and electrostatically charged powder is sprayed towards the cores to form a powder coating attached to the surfaces of the cores. The powder coating is then fused to give a fused film coating secured to the core.
A disadvantage of such a method is that the majority of the powder sprayed towards the cores is not charged and is not deposited on the cores. That leads to overspray and to wastage of the powder material and makes it difficult to obtain a uniform coating.